Communications systems have need for components, such as resonators and filters, that control frequency generation or that limit the range of frequencies used in the systems. Resonators are not only formed of electrical inductor and capacitor circuits, but can also be electromechanical in nature, such as quartz crystals, surface acoustic wave devices, or thin film bulk acoustic resonators. Frequency filters also may be based upon that core resonator technology. Resonators exhibiting multiple resonances at nearby frequencies within a single structure would be particularly useful in frequency filters to produce multiple pass-bands and/or stop-bands of frequency. Both the resonators and the filters formed with those resonators may take advantage of the piezoelectric type of resonator, a form of electromechanical resonator.
In the simplest form, a piezoelectric electromechanical resonator is composed of a piezoelectric plate sandwiched between a pair of electrodes formed in a unitary assembly. Resonator 1, illustrated in FIG. 1a, representative of prior art to which reference is made, is composed of a piezoelectric plate 2 that possesses sufficiently aligned and smooth surfaces 3 and 4 to which metal plates or electrodes 5 and 6 are respectively attached. An electromechanical thickness-mode resonance is established by the piezoelectric transduction process, wherein electrodes 5 and 6 are electrically driven by a signal of such frequency that an acoustical standing wave is established across the thickness dimension of the piezoelectric plate, perpendicular to the plane of the electrodes. The acoustic deformation produced in the foregoing transduction propagates in a direction that is substantially normal to the piezoelectric plate and the electrodes, and is referred to herein as a bulk acoustic wave or simply a bulk wave. The bulk wave may be an acoustic shear wave which exhibits mechanical deformation in directions that are substantially transverse to the direction of propagation or an acoustic longitudinal wave that exhibits mechanical deformations in a direction that is substantially parallel to the direction of propagation.
The fundamental (e.g. the lowest frequency) resonance of the plate occurs when a half wavelength of standing acoustic wave is produced between the outer surfaces 7 and 8 of respective electrodes 5 and 6. Other higher-mode resonances occur at those higher frequencies at which an integral number of half-wavelengths of acoustic vibration exists between surfaces 7 and 8. Resonators having perfect physical symmetry, as suggested in FIG. 1a, exhibit only resonances at odd multiples of the fundamental frequency because of the coincidence of the odd symmetry of the applied voltage (“+” on one electrode, “−” on the other electrode) and the structural symmetry of the device. If the resonator is not structurally symmetric, then resonances may also occur at even order multiples of the fundamental frequency. In either case, the resonances are separated in frequency by a relatively wide margin, a margin that is approximately equal to the fundamental resonant frequency of the device.
In literature describing the prior art, the word resonator is sometimes used in two different senses creating a possible ambiguity. The first is to refer to that portion of the device that exhibits substantial mechanical vibratory deformations (i.e. acoustic vibrations) when periodically perturbed with a mechanical (acoustical) force at rates or frequencies near the mechanical resonant frequency of that portion of the device. Second, that literature also sometimes uses the word resonator to refer to the entire device. To avoid such an ambiguity, in this specification, unless the context indicates otherwise, the term “mechanical resonator” is intended to refer to a portion of the entire device that exhibits substantial acoustic vibrations. The term “resonator device” will refer to the entire device that contains that mechanical resonator together with a transducer, and that transducer may also function in the device as a second resonator. Further, with regard to the present invention, the term “resonator device or the term “dual-frequency resonator device” means the entire device, unless the context indicates otherwise.
Some additional definitions should be helpful to more quickly achieve an understanding of the invention. The term “transducer/resonator” usually means that portion of the acoustic resonator device that exhibits substantial acoustic vibrations near resonance and in which the transducer physically constitutes all or a substantial part of the transducer/resonator. The transducer is piezoelectric in character and includes electrically conductive electrodes or terminals for applying electrical voltages to (or extracting electrical voltages from) the piezoelectric material. The term “mechanical resonator”, as earlier described, usually refers to a portion of a resonator device that exhibits substantial acoustic vibrations when perturbed at a frequency near the frequency of resonance of that portion of the device. The mechanical resonator does not include a transducer and is physically separate from the transducer/resonator, even though the mechanical resonator may be acoustically coupled to that transducer/resonator.
For clarity of illustration, the figures greatly exaggerate the thickness of the layers relative to the lateral extent of the layers. For example, in the bulk-wave piezoelectric resonator device of the type depicted in FIG. 1a, the lateral dimension of the electrodes, typically, will be of the order of fifty or so times greater than the thickness of the layer of piezoelectric material. Although not depicted in FIG. 1a, the piezoelectric layer also may extend laterally substantially beyond the right and left-hand boundaries of the electrodes. These extensions of the piezoelectric layer may be used to mechanically support the resonator device.
An acoustic device can include two or more portions that form resonators. Each of these portions exhibits an acoustic resonance. If one resonator portion is acoustically coupled to a second resonator portion, the two resonator portions interact. The interaction is similar to the effect that occurs with electrical coupling between two resonant electrical circuits (e.g. two circuits each circuit consisting of a series connection of an inductance and a capacitance) that are, individually, resonant at the same frequency. In such coupled electrical circuits, when one compares the magnitude of an electrical signal input to one resonant circuit to the electrical signal output from the second resonant circuit, as a function of frequency, one observes a narrow peak in the output signal near the resonant frequency of the two circuits when the electrical coupling between these two circuits is small, which level of coupling is commonly referred to as “under-coupled.” As the electrical coupling is increased, the peak in the output signal increases in level and broadens out as a function of frequency. As the coupling is further increased the peak in the output signal versus frequency eventually broadens and splits into two separate peaks, peaked at different frequencies, with a dip in signal level between the two peaks. This circumstance is commonly referred to as “over-coupled.” At the level of coupling where the peak is the broadest but does not yet exhibit a central dip, the circuits are said to be “critically-coupled.” Acoustically coupled resonators exhibit similar effects.
Resonators of the conventional form (of FIG. 1a) are also known to exhibit additional resonances due to standing waves distributed parallel to the principal plate (e.g. electrode) surfaces 7 and 8. These resonances are formed by plate wave reflections at the edge of the electrodes or mounting structures. Such resonances are generally unwanted or spurious and are not used, except in some specialized applications. These resonances are normally avoided by using electrodes whose lateral dimensions are many times greater than the thickness of the piezoelectric layer. In a structure similar to that depicted in FIG. 1a, but in which the lateral extent of the electrodes is of the order of the plate thickness, the acoustic resonances are two dimensional in nature. Due to the construction of such a plate wave resonator, the lateral extent of the acoustic vibrations are substantially confined, or “trapped”, by the width of the electrodes or some other physical feature. However, if two such resonators are placed in close lateral proximity, a small portion of the acoustic vibrations in the first resonator are acoustically coupled to the second resonator and induce some acoustical vibrations in the second resonator. In that configuration the lateral spacing of the two resonators controls the amount of acoustic coupling.
So-called monolithic, two-port, crystal filters have been fabricated from two or more “trapped energy” resonators placed in close lateral proximity. Wave coupling lateral to the plate thickness direction produces a split in the resonance and thus a multi-pole filter response when one resonator is driven and the other resonator is loaded into a circuit. The resonators are placed side-by-side and the coupling is perpendicular to the primary thickness resonance direction. The degree of resonator coupling and number of resonators affects the overall response.
In contrast to the side-by-side configuration of resonators in the filter described in the preceding paragraph, the present invention utilizes a transducer/resonator and a mechanical resonator that are vertically stacked, instead of being placed side by side, and, in the present invention, the transducer/resonator is separated from the mechanical resonator by layers of material that function as an acoustic coupler. The layers of the acoustic coupler are selected so as to obtain a desired degree of acoustic coupling between the transducer/resonator and the mechanical resonator. Additionally, the present invention is a single-port device.
In this specification the term “transducer” is intended to refer to a device (or that portion of a device) that converts electrical signals into mechanical vibrations (and vice-versa). The term “transduction” refers to the physical process of converting or transforming electrical signals into mechanical vibrations (and vice-versa). Thus, in the prior art transducer of FIG. 1a, the transducer is composed of electrodes 5 and 6 and piezoelectric material 2, and the resonator is bounded by surfaces 7 and 8 that reflect acoustic waves generated by the transduction process. That confines most of the acoustic vibrations to the region bounded by surfaces 7 and 8. In this simple case, the transducer constitutes the entire resonator portion of the device and is referred to herein as the transducer/resonator.
Reference is made to FIG. 1b, labeled “prior art”. FIG. 1b illustrates an acoustic resonator device that constitutes a variation of the simple, thickness-mode plate resonator of FIG. 1a. Here, transducer 51 is attached to layer 59 in a vertically stacked unitary assembly so that the combination forms a resonator device 50. At resonance, a half wavelength (or appropriate multiple of a half wavelength) acoustic wave is produced between surfaces 57 and 60. Although transducer 51 may, alone, be substantially less than a one-half (acoustic) wavelength in thickness, the distance between surface 57, the upper surface of the transducer, and surface 60, the bottom surface of layer 59 comprises one-half acoustic wave-length or a multiple thereof. The principal acoustic vibrations occur between surfaces 57 and 60. The combination of transducer and layer 59, which together have an acoustic thickness of one-half acoustic wavelength, is also referred to herein as a “transducer/resonator.” It should be remembered, however, that in many instances the transducer may constitute all of the resonator.
As a further variation of the foregoing configuration, transducer 51, alone, may be a half wavelength thick, and layer 59 is then a half wavelength thick or an integral multiple thereof. In this latter case these resonator devices are conventionally referred to as “overmoded” resonators, see K. M. Lakin, G. R. Kline and K. T. McCarron, “High Q Microwave Acoustic Resonators and Filters”, IEEE Trans. Microwave Theory Tech. Vol. 41 no. 12, December 1993, pp. 2139–2146.
A further variation on the thickness mode plate resonator device is shown in FIG. 2. Here a single port, resonator device 90 comprises substrate 80, acoustic isolator 89, and transducer/resonator 79 in a vertically stacked integral (e.g. unitary) assembly. Substrate 80 is the general support structure and may consist of any of a wide variety of materials, such as silicon, alumina, sapphire, or other material, generally in a wafer form that is compatible with integrated circuit processing techniques. Acoustic isolator 89 may consist of a single layer of material or of a sequence of quarter wavelength thick layers of material, illustrated in this example as layers 81 through 85, in sufficient numbers and having sufficient relative impedance discontinuities between adjacent layers, to thereby acoustically isolate transducer 79 from substrate 80. Such layers are often designed to have alternating extremes of acoustic impedance such that significant wave reflections occur at surfaces 91 through 96 such that little or no vibrations reach substrate 80. Typically layers of high impedance materials such as aluminum nitride (“AlN”) or tungsten (“W”) alternate with layers of low impedance materials such as silicon dioxide (“SiO2”). An acoustic isolator is sometimes referred to herein as a reflector.
Transducer 79 is composed of piezoelectric region 87 and interfaces 97 and 98, to which are attached associated electrodes 86 and 88. Electrodes 86 and 88 include external points of electrical connection to other circuitry, not illustrated. Transducer 79 also serves as a resonator. The acoustic vibrations at resonance are substantially confined between surface 96, i.e. the inner or lower boundary of the transducer, and the air or vacuum interface at surface 99, i.e. the outer or upper surface of the transducer, in a manner similar to that of the conventional resonator illustrated in FIG. 1a. 
The acoustic vibrations in device 80 rapidly diminish with distance from resonator 86 within the isolator reflector array 81 through 85, as illustrated in FIG. 3, to which reference is made. In FIG. 3 transducer region 79, which is about 4.00 μm thick at 1600 mhz, shows a half wavelength of acoustic vibration. The standing wave rapidly diminishes in amplitude throughout acoustic isolator 89 with little wave amplitude reaching substrate 80. For this example, electrodes 86 and 88 are each 0.3 μmeters of aluminum, piezoelectric material 87 is 3.0 μmeters of AlN, with the reflector sequence starting with 0.7 μm of SiO2 and then 1.76 μmeters of AlN alternating until the final layer of SiO2 is reached. The wave distribution was calculated for 1600 MHz with +1.0 Volt on electrode 88 and −1.0 Volt on electrode 86.
In contrast to the resonator devices of FIGS. 1a and 1b, the graph of the amplitude of acoustic vibration vs distance presented in FIG. 3 illustrates that the acoustic vibrations in resonator device 90 of FIG. 2 extend beyond the boundaries of transducer/resonator 79 and, in fact, penetrate into isolator 89. Because the acoustic vibrations are not entirely confined to transducer/resonator 79, and because a small portion of these vibrations extend into the isolator, the electrical and acoustic properties of the device are, to some extent, affected by the presence of and the acoustical properties of the layers of material that constitute the isolator. Resonators of the type described in FIG. 2 have been disclosed in U.S. Pat. Nos. 3,414,832 and 5,373,268 and 5,821,833.
Because the present invention possesses more complex properties, to aid in understanding the present disclosure, electrical resonances are defined in terms of the electrical impedance of the structure rather than by the more conventional half-wavelength or frequency-thickness descriptions. The electrical characteristics of a piezoelectric resonator can be described by the electrical impedance of the input port, i.e. at the input terminals to the electrodes, and in a manner analogous to well known inductor-capacitor resonant circuits. Most useful in identifying and defining the meaning of electrical resonance in such a one-port device is the nature of the phase of the electrical input impedance, Zin. as a function of frequency, f. As an example, reference is made to FIG. 4 showing the computed phase and magnitude of the electrical input impedance of a resonator near the fundamental resonant frequency of approximately 1600 MHz. The magnitude of the input impedance is shown as a dashed line and the phase of the input impedance is shown as a solid line. At frequencies 120 below electrical resonance, the phase is near −90 degrees (lagging), which is analogous to a capacitive reactance. At frequency 121, the phase slope is positive, the phase is zero degrees, and the impedance is purely resistive and at a minimum value. The conditions of positive phase slope, zero phase, and minimum impedance characterize series resonance.
Between frequencies 121 and 123, the phase 124 is positive, analogous to an inductance. At frequency 123 the phase slope is negative, the phase is zero, and the impedance is resistive and of high value relative to the resistive impedance at 121. The impedance conditions at 123 characterize parallel resonance. At higher frequencies 125, the phase is again negative, analogous to a capacitive reactance. For the purpose of this specification, series resonance means the frequency at which the input impedance exhibits zero phase and a positive phase slope with a relatively low value of resistive impedance. In contrast, parallel resonance means a frequency at which the input impedance exhibits zero phase, negative phase slope and a relatively high resistive impedance.
A simple resonator of the prior art whose input impedance exhibits an input phase that undergoes a phase excursion from approximately −90 degrees to approximately +90 degrees and then back to approximately −90 degrees as the frequency increases, such as that depicted in FIG. 4, is referred to in this specification as a single-frequency electrical resonator. The single frequency resonator exhibits an electrical resonance that includes both a series resonance and a parallel resonance at a slightly higher frequency. For the purpose of this specification, the frequency increment between series resonance, 121, and parallel resonance 123 is defined as the resonator bandwidth. The mean of the series resonance frequency and the parallel resonance frequency is referred to as the nominal resonant frequency. Although the prior art device depicted in FIG. 1a is referred to as a single-frequency electrical resonator, the device exhibits similar phase excursions at each a series of the harmonic electrical resonance's. Those harmonic electrical resonances occur at approximately odd integer multiples of the frequency of the fundamental electrical resonance for the simple resonator.
A purely mechanical resonator possesses resonances that are defined by peaks in acoustic standing wave amplitude. The concept of electrical series or parallel resonance is inapplicable to mechanical resonators. The frequency at which a mechanical resonator, in the absence of acoustic coupling to other portions of the device, would exhibit a peak in the amplitude of the acoustic standing wave is referred to herein as the mechanical resonant frequency of the isolated mechanical resonator.
Methods for fabrication of piezoelectric resonators for use at microwave frequencies are well known in the prior art. See, e.g., the descriptions of such devices in the specification of U.S. Pat. No. 5,894,647 for a “Method for Fabricating Piezoelectric Resonators and Product”, Lakin, and see the references to prior art cited therein. See also “Microwave Acoustic Resonators and Filters,” by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139; Guttwein, Ballato and Lubaszek, U.S. Pat. No. 3,694,677; and “Acoustic Bulk Wave Composite Resonators”, Applied Physics Letters 38(3) by Lakin and Wang, Feb. 1, 1981. Such resonators also may be fabricated on, and supported by, a substrate by including a set of intervening layers of material having alternating high and low acoustic impedances, which layers have thickness' of a quarter wavelength. The intervening layers act as an acoustic mirror or reflector that acoustically isolates the resonator from the underlying substrate. See, e.g., U.S. Pat. Nos. 3,414,832 and 5,373,268 and 5,821,833 and 6,291,931. For methods of analysis and further descriptions of reflectors and resonators see Lakin, “Solidly Mounted Resonators and Filters, 1995 IEEE Proc. Ultrasonics Symposium, pp. 905–908 and Lakin et al. “Development of Miniature Filters for Wireless Applications”, IEEE Trans. on Microwave Theory and Techniques, Vol. 43, No. 12, December 1996, pp. 2933–2939. It is expressly understood that the content of the foregoing prior art publications, as well as the content of any other prior art cited herein by reference to a publication or a patent is incorporated herein by reference in the entirety.
The present invention relates to single-port microwave acoustic resonator devices and, more particularly, to obtaining electrical resonances at the input impedance of the single-port electro-acoustic resonator device that have a more diverse range of properties or that are more complex than the properties exhibited by a simple, single resonator device. That result is obtained using techniques of controlling the resonant frequencies of the transducer/resonator and of the mechanical resonator and by controlling the amount of acoustic coupling between the transducer/resonator and the mechanical resonator.
Accordingly, a principal object of the invention is to provide a single port, piezoelectric resonator device, useful at microwave frequencies, whose input impedance exhibits a more complex and diverse character than can be obtained from a prior art one-port device comprising a simple transducer/resonator.
A further object of the invention is to design a single port, piezoelectric resonator device, useful at microwave frequencies, whose input impedance exhibits two or more electrical resonances that are not harmonically related and are located relatively close together in frequency.
A further object of the invention to provide in a one-port microwave acoustic resonator device whose input impedance exhibits at least two frequencies of electrical resonance that are spaced apart in the frequency spectrum by a fractional increment of the first resonance.